Method of making thin-walled refractory structures



3,473,987 METHOD 8F MAKING THIN-WALLED REFRACTGRY STRUQTURES Donald Maurice Sowards, Claymont, DeL, assignor 10 E. I. du Pont de Nemours and Company, Wilmington, Bet, a corporation of Delaware No Drawing. Filed July 13, 1965, Ser. No. 471,738 lint. Cl. 332i) 3/12; C0412 35/10 Cl. 156-89 5 Claims ABSTRACT OF THE lDlISCLOSURE A process for the manufacture of thin-walled aluminacontaining refractory structures which have improved strength characteristics as compared with structures of similar chemical composition known in the art. Said process is particularly directed to making these improved structures by oxidizing metallic aluminum template structures coated with finely divided aluminum powder and a fiuxing agent.

The firing of an aluminum template structure such as an aluminum honeycomb coated with a fluxing agent to produce an alumina-containing refractory structure is taught in Belgian Patent 612,535, granted July 11, 1962. Thin-walled structures made by the process of the Belgian patent have excellent strength and good thermal shock resistance.

The process of the Belgian patent results in structures having double walls. These double walls result from the fact that the aluminum as it melts apparently migrates outwardly through fissures in the oxide film formed n the surface of the metal and is oxidized on the outer surface of the oxide film. Thus the oxidation of a sheet of aluminum foil according to the process of the Belgian patent results in a material having a large void between two aluminacontaining walls. The void corresponds substantially in thickness to the thickness of the original aluminum sheet. The thickness of each of the aluminacontaining Walls is generally less than the thickness of the original aluminum sheet.

Increasing the thickness of the walls of a thin-walled ceramic structure would ordinarily be expected to increase the strength of the structure proportionately. However, due to the nature of the process of the Belgian patent, the thickness of the walls can be increased only by increasing the thickness of the aluminum in the template structure. Increase in the thickness of the aluminum also results in an increase of the thickness of the void between walls. Thus, any gain in strength due to the increased thickness of the walls is to a large extent offset by loss in strength due to increased thickness of the void.

It has now been found that thin-walled alumina-containing refractory structures having improved strength characteristics can be produced by the in situ oxidation of aluminum template structures in the presence of a fluxing agent if the template is coated prior to firing with finely divided aluminum powder. In the preferred practice the template is also coated with a finely divided filler refractory in addition to the aluminum powder. The structures produced by this process have the double walls characteristic of the products of the Belgian patent, but the use of the aluminum powder results in increased thickness of the alumina-containing walls in relation to the thickness of the void between the walls and thus provides a stronger product.

The aluminum template structure can have any desired shape. The process of the invention faithfully reproduces the shape of the template. Examples of structures which can be made by this process are cans, tubes, boxes, arrays hired States Patent 0 "ice of tubes, honeycombs, and many other more or less complex shapes.

The aluminum template structures can be fabricated from aluminum sheets or can be formed by extrusion methods. The aluminum need not be of a high degree of purity. Alloys in which aluminum constitutes the major part can also be used. The Walls of the aluminum template structure will ordinarily have a thickness between about 1 mil and about 35 mils. Since it is desirable to make structures in which the thickness of the void between the walls is small, it is preferred to use aluminum sections having a maximum thickness of about 10 mils. The most useful products are made from aluminum having a thickness between about 1 mil and about 5 mils. The aluminum section can be solid, perforated, or foamed.

The process requires the presence of a fiuxing agent. Suitable fiuxing agents for use include the oxides of the alkali metals, the alkaline earth metals, vanadium, chromium, molybdenum, tungsten, copper, silver, zinc, antimony and bismuth. Precursors of these oxides and hydroxides of the alkali metals can also be used. The oxides and hydroxides of the alkali metals, magnesium, strontium, and barium are preferred.

Among suitable precursors of these materials may be mentioned the organic salts such as acetate, benzoate, and the like, and inorganic salts such as bisulfates, bisulfites, bromates, nitrates, silicates, sulfates, sulfites, and thiosulfates of the recited metals. While not per se within the class of useful fiuxing agents, these compounds do under conditions of the reaction yield oxides within the above defined class. In addition trialkyl tin oxide and lead silicate are also useful as fiuxing agents. The preferred fluxing agent is sodium silicate which yields sodium oxide on firing.

Although applicant does not wish to be bound by any theory, it is believed that under the conditions of the oxidation, the fluxing agents serve to cause fissures in the oxide film formed on the outer surface of the aluminum, thus allowing the aluminum to flow outwardly. They also apparently serve as Wetting agents, permitting the aluminum, as it flows through the fissures, to b spread over the surface of the oxide film where it is oxidized.

The amount of fluxing agent to be used is not particularly critical. Ordinarily the amount will range between about ().02 and 29% based on the total weight of aluminum in the template structure and in the coating. Preferably about 0.2 to 5% is used. The fluxing agent is calculated on the basis of the metal oxide that is for-med in those cases where a metal oxide precursor is used. Higher concentrations of fiuxing agent may be employed but are generally avoided except in those cases where the fluxing agent also acts as a filler refractory to prevent undue lowering of the melting point of the final structure and loss of strength at elevated temperatures.

The aluminum powder, like the aluminum template, need not be of a high degree of purity and alloys of aluminum in which aluminum constitutes the major proportion can also be used.

As mentioned above, a filler refractory can be used in addition to aluminum powder in order to increase the thickness of the walls of the structures of the invention in relation to the thickness of the central voids. Generally the suitable refractories are any of the carbides of silicon, aluminum, boron, hafnium, niobium, tantalum, thorium, titanium, tungsten, vanadium, and zirconium; the nitrides of aluminum, boron, hafnium, niobium, tantalum, thorium, titanium, uranium, vanadium and zirconium; the borides of chromium, hafnium, molybdenum, niobium, tantalum, titanium, tungsten, vanadium and zirconium; or the oxides of aluminum, beryllium, cerium.

chromium, hafnium, iron, lanthanum, magnesium, nickel, titanium, cobalt, manganese, thorium, copper, uranium, yttrium, the stabilized oxide of zirconium, and silicon dioxide.

Precursors and mixtures of these refractory compounds and compounds containing these refractory materials can also be used. For example, various clays such as kaolin, ball clay, and the many fire clays are satisfactory. Burnt clays (i.e. grog) may be used. Minerals containing magnesium and silicon such as asbestos, talc, steatite, soapstone, fosterite, and vermiculite are satisfactory.

Certain of the filler refractories may react with alumina formed by the in situ oxidation of the aluminum template and aluminum powder. For example, magnesia may react to form spinel, silica may react to form mullite, and a mixture of silica and magnesia may react to form cordierite. The preferred structures of this invention consist essentially of a-alumina and are prepared by firing an aluminum template coated with about equal parts by weight of Al powder and alumina.

The aluminum powder plus refractory filler, if any, to be used, in relation to the amount of aluminum in the template, will depend upon the desired thickness of the walls relative to the thickness of the void between the walls in the final structure. Ordinarily there should be used between about and about 12 parts by weight of the powder plus refractory, if any. Use of these amounts will ordinarily provide structures in which the wall thickness exceeds the void thickness by a factor in the range of about 2:1 to about 60: 1. Lower amounts of aluminum powder plus refractory can of course be used but do not provide bodies of greatly improved strength over the structures of the Belgian patent referred to above. Larger amounts can be used but as the thickness of the coating increases it becomes progressively more difiicult to achieve complete oxidation without inclusion of a combustible organic material which will decompose upon firing to provide porosity. This can be done but is not preferred since the added porosity adversely affects the strength of the final structures. Preferably, the amount of powder plus refractory will be in the range of about 1 /2 to about 6 parts per part of aluminum in the template, so that the ratio of wall thickness to void thickness will be in the range of about 4:1 to about 30:1, particularly in making rather small or complex structures using a template with Al sections less than about 10 mils thick.

It should be observed that the ratio of Wall thickness to void thickness in any given structure will depend not only upon the ratio of coating (i.e. aluminum powder plus filler refractory, if any) to aluminum in the template but also upon the bulk density of the coating and the amount of shrinkage or expansion upon firing. The values stated are estimates based on observations made in using typical coatings in accordance With this invention. It should also be observed that .as the thickness of the walls of the aluminum template increase, it becomes impractical to use as much as say 12 parts of coating per part of aluminum in the template, since the thickness of the coating is so great that adequate oxidation is difi'icult to achieve without inclusion in the coating of a decomposable material. Thus as a practical matter, when a template having walls of say 35 mils in thickness is used the ratio of the weight of aluminum powder or mixture of aluminum powder plus filler refractory to aluminum in the template should be no more than about 3 or 4. Thus, the thickness of the walls in the final structures will exceed the thickness of the voids by a factor of only about 15:1 or less. In other words when a template having 35 mil thick walls is used, the individual walls in the final structure will be no more than about /2 inch in thickness so that the overall thickness of the sections will not exceed about 1 inch.

At least about 15% by Weight of the total weight of aluminum powder plus refractory should be aluminum powder. Thus, the minimum amount of aluminum powder to be used will be about 0.1 part per part of aluminum in the template which occurs in the case where the ratio of powder plus refractory to template is about and the amount of powder in the powder plus refractory is 15%. The maximum, of course, will be about 12 parts powder per part of aluminum in the template, since there need not be used any filler refractory in the coating.

Where a filler refractory is used which reacts or forms a solid solution with alumina the amount of aluminum powder in the powder plus refractory will be at least about 15 regardless of the amount of coating used in relation to the amount of template. Thus, where for each part of aluminum in the template there are used 12 parts by weight of aluminum powder plus reactive filler refractory, e.g. magnesia, the minimum amount of Al powder will be about 1.8 parts and the maximum amount of filler refractory will be about 10.2 parts. The maximum ratio of the weight of reactive filler to total aluminum. i.e. that in the powder plus that in the template will thus be about 4: 1.

On the other hand, when a filler refractory is used which is not reactive with the alumina, there should not be used more than about 1.5 parts of it per part of total aluminum, due to inadequacy of the bonding provided by the in situ formed alumina at higher ratios. This means that where the coating (i.e. aluminum powder plus refractory) is used in an amount of 12 parts per part of template aluminum, the amount of powder in the powder plus refractory mixture should be at least about 35% by weight. As the ratio of weight of powder plus non-reactive refractory to template aluminum decreases to say about 2.5 the required percentage of powder in the powder plus refractory may decrease to about 15%, the only requirement being that the ratio of weight of non-reactive filler to total aluminum in the coated structure not exceed about 1.5.

Certain of the filler refractories listed above may also serve as fluxing agents in the process. For example, magnesia is a fluxing agent and it is possible, although not preferred, when using magnesia to dispense with any other fluxing agent in the coating composition. Similarly, certain clays may contain sufiicient alkali metal oxides to serve as a flux.

The aluminum powder, fluxing agent, and filler refractory, if any, are most conveniently applied simultaneously to the aluminum template structure. For this purpose the components can be dispersed or slurried in water or in an organic solvent. The coating may then be applied to the structure 'by dipping, brushing, spraying or any other conventional means. The coating may also be applied to aluminum sheets which are fabricated to the desired structure after coating.

After the coating step the structure is dried for a period of several minutes up to about an hour in a forced draft ambient air. Normally a period of about three quarters of an hour is sufiicient for thoroughly drying products. When the structure is thoroughly dried it is ordinarily desirable to set the coating by heating for a few minutes at a temperature of about 170-250 C. For a structure such as a dry, coated aluminum honeycomb shape, which is susceptible to warping during the heat setting step, it is sometimes desirable to press the structure at a few pounds pressure between platens for a few minutes.

The coating, drying and heat setting steps are repeated until the desired amount of aluminum powder, fluxing agent and filler refractory have been deposited upon the template structure. In most instances two coating, drying and heat-setting cycles are sulficient.

The size of the aluminum particles used in the process is not particularly critical, but it is preferred that they be sufficiently small to serve as a thickening agent for the coating compositions. Thus, particles in the range of about '325 to 16 mesh will ordinarily be used and particles of about -325 to mesh are preferred to produce a stable, coherent coating. Similarly the particles of filler refractory will preferably be less than about 16 mesh and will ordinarily be between about 325 and l00 mesh. With much larger particles it is difiicult to obtain a continuous coherent coating on the template structure.

The addition of green binding agents to the coating composition is often advantageous and frequently necessary with coating compositions that do not provide adequate adhesion until firing temperatures are reached. Substances such as clay, sodium alginate, sodium carboxymethylcellulose, natural gums, polysaccharides, synthetic resins such as polyvinyl alcohol and polyacrylic acid and the like are suitable.

Where a solution of the preferred fiuxing agent (i.e. solium silicate) is used as a base for the coating composition a drying step following the coating step leaves a firm adherent film of sodium silicate that is sufiiciently refractory to remain in place during the critical part of the firing procedure. Where a catalyst is used which does not produce this firm, adherent film, ceramic products such as Portland cement, Sorel cement and the like can be added to the coating composition. These products in combination with the green binding agents mentioned above will, upon the removal of water, provide a stable adherent coating or film.

After the template structures have been coated, dried and heat set they are then fired in an oxygen-containing atmosphere to oxidize the aluminum. The firing temperature should preferably be at least about 660 C., which is the melting point of aluminum. Lower temperatures, say as low as 400 C., can be used but at these temperatures the oxidation proceeds extremely slowly. Firing of the structure should continue until at least about 60% by Weight of the aluminum in the coated template has been oxidized. It is preferred to continue the firing until substantially all of the aluminum has been oxidized. In order to achieve substantially complete oxidation it is ordinarily necessary to raise the firing temperature in the latter stages of the cycle to above 1000 C. and preferably above 1400 C.

The time required for oxidation of course depends upon the temperature at which the structure is fired and also upon the thickness of the coating and of the sections in the original aluminum template. Structures of cross sections near one inch may require firing for as much as 150 hours or longer at temperatures well above 1000 C. in order to achieve substantially complete oxidation.

It should be observed that it is desirable to carry out the oxidation gradually in order to prevent ignition of the aluminum which may result in badly cracked and crazed bodies. Thus, it is preferred to carry out a substantial proportion of the oxidation at temperatures below about 850 C. and to raise the temperature above 1000 C. only in the latter stages of the oxidation. The gradual heating is more critical in the case of structures of relatively large cross-section than in these of small crosssection, due to the greater volume per unit surface area and consequently the greater heat retention in the former than the latter. In general, it can be said the rate of heating should be controlled such that the temperature of the structure at no time during firing exceeds the ambient temperature by more than 200 C. Preferably, the rate is such that a temperature differential greater than 50 C. does not occur.

In a typical firing procedure the structure is placed into a cold furnace and the temperature is raised gradually to about 1600" C. in a period of about 48 to 60 hours. The gradual heating provides a coherent structure and prevents excessive migration of the aluminum whereby large voids are formed and/ or aluminum melts out of the structure. If desired, the structure can be further fired at a temperature of about 1600 C. for another 36 to 48 hours in order to substantially complete the oxidation of the aluminum and to form compounds such as spinel, mullite or cordierite.

Considerable leeway in the firing cycle is possible. Those skilled in the art will readily be able to determine the optimum firing procedure for any given structure and composition.

DESCRIPTION OF THE PRODUCTS The products of this invention are thin-walled refractory structures composed of integrated sections of refractory material. The principal constituent of the refractory material is a crystalline alumina-containing phase which extends in a continuous network or skeleton throughout the section and in fact throughout the entire structure. In describing the structure as being composed of integrated sections, I mean that the points of contact between sections of refractory materal are made up of a continuum of this crystalline alumina-containing phase.

This alumina-containing phase can be substantially pure alumina or it can be a compound or solid solution of alumina and at least one other metal oxide or a solid solution of at least one metal oxide in said compounds of alumina. Any metal oxide (including the oxides of silicon and boron, sometimes referred to as metalloids) can be associated with alumina as a chemical compound or solid solution and thus appear in the crystalline skeletal structure. Thus the oxides in this skeleton, in addition to alumina, may include the oxides of magnesium, silicon, chromium, the alkaline earth metals, the alkali metals, titanium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, lanthanum, hafnium, tantalum, tungsten, cerium, thorium and the like. The aluminum oxide either in free or combined form will comprise at least 30% by weight of this continuous phase.

If a filler refractory is employed which does not form a compound or solid solution with alumina, such as a boride, carbide or nitride, then the refractory material in addition to the skeleton of alumina-containing refractory will contain the filler refractory in unaltered form.

The alumina-containing skeletal phase in the structures of this invention contain enclosed pores or cells having an average diameter in the range of about 10 microns to microns. These pores result either from the in situ oxidation of the aluminum powder in the coating composition or from porosity in the coating before firing. The pores are substantially uniformly distributed throughout the crystalline alumina-containing skeleton. In a given structure the average diameter of the pores will correspond roughly to the average diameter of the particles of aluminum powder used to coat the aluminum template.

The size of the pores or cells Within the walls is determined by the lineal analysis of microstructure technique as discussed by W. D. Kingery in Introduction to Ceramics, pages 4l2417 (published 'by John Wiley and Sons, Inc., New York, 1960). The individual cells in the alumina-containing phase of the products of this invention may have diameters varying from about 0.5 micron up to 200 or 300 microns or larger, depending upon the particle size of the aluminum powder used. There may be pores even smaller than 0.5 micron in diameter, but the number of such pores is extremely small, and they are ignored in determining average cell (pore) size.

The grain size in the crystalline alumina-containing skeleton is not uniform. The average grain size is greater than 0.5 micron as evidenced by the fact that no line broadening is observed in an X-ray diffraction pattern of the products. Areas have been observed in certain products of the invention where the average grain size is of the order of 0.2 to 0.4 micron but these areas are minor. In general, the average grain size lies between about 2 and about 8 microns, although much larger grains have been observed.

The overall thickness of the sections of refractory material making up the integral structures of this invention will range from about 5 mils to about 1 inch or slightly higher depending upon the thickness of aluminum in the template and the thickness of the coating in the coated but unfired structure. In the preferred products, the thickness of the sections will range from about mils to about 300 mils.

The sections of refractory have a large sheet-like void near center which extends substantially the entire length and width of the section. This void results from the oxi- ,dation of the aluminum in the template and corresponds substantially in thickness to the thickness of the aluminum sections in the template. Thus the thickness will generally range from about 1 to about 35 mils and preferably from about 1 to about 10 mils.

In some products of the invention there will be occasional bridges of refractory material across the sheetlike voids. In some, especially those made from about 1 mil thick aluminum foil template, the void may be sintered closed or so thin as to be barely discernible under the light microscope.

As stated above it is preferred that the thickness of the walls in the double-wal1ed refractory sections exceed the thickness of the sheet-like void between the walls by a factor between about 2 and about 60 and preferably by a factor between about 4 and about 30. When made using a template constructed of very thin foil the wall thickness may of course appear to be much more than 60 times the void thickness since the void may be sintered closed or almost closed and may be barely visible under the light microscope. Further, as explained above when a template made up of aluminum sections of more than about 10 mils in thickness is used the maximum ratio of wall thickness to void thickness will ordinarily be less than 60. For example when the template is made up of about 35 mil thick sections, the Walls in the final structure will ordinarily be no more than about /2 inch in thickness, i.e. the maximum ratio of wall thickness to void thickness will be no more than about :1, since it is not practical in this instance to use sufficient coating to give a structure in which the ratio is higher. Where, however, a template made out of say 3-l0 mil aluminum foil is used the full range of coating/template ratios can be used, and the products will in general have ratios of wall thickness to void thickness of from 2 to 60, as stated.

The structures produced according to the present invention have utility as high temperature insulation panels, heat exchangers, radiant burner elements, and catalyst carriers and supports.

The invention will be further described by the following illustrative examples. In the examples as well as in the description above all sieve measurements are made with US. Sieve Series. The expression X mesh indicates that the particles are substantially all passed through an X mesh sieve. The expression X/Y mesh indicates that all particles pass through an X mesh seive and are retained on a Y mesh sieve.

'Example 1 An 11" x 11" x 1" aluminum honeycomb of W cell size made of 0.002" aluminum foil of 5052 alloy obtained from the American Cyanamid Company is etched in 1% caustic solution to a weight of 255 grams. A slurry containing:

1 part by weight of a 1% solution of earboxymethylcellulose (CMC), (Hercules Type 7HS);

1 part by weight of sodium silicate solution, 41 B.,

NagOzSiO 1:3.25 (DuPont Type 9 Technical);

0.5 part by weight of aluminum powder (Alcoa 123, 325

mesh);

0.7 part by weight of 100 mesh alumina powder (Alcoa Calcined Type A-S);

1.5 parts by weight of 325 mesh silicon carbide powder (Norton Type F Abrasive Grain);

0.5 part by weight of Cedar Heights bonding clay as received from Cedar Heights Clay Company, Oak Hill, Ohio;

0.5 part by weight of water is prepared, and the etched aluminum honeycomb 1s coated by immersing in the slurry. After coating, the aluminum honeycomb is dried at room temperature and heat set by pressing lightly between platens heated to 200 C. for 5 minutes. The final weight of the slurrycoated aluminum honeycomb template is 490 grams. Two hundred thirty-five grams of solids are thus applied or 0.92 part by Weight per part of aluminum template, 0.10 part sodium silicate, 0.13 part aluminum powder. 0.18 part alumina, 0.42 part silicon carbide and 0.14 part clay. The unfired structure contains 0.8% sodium silicate Na O as fluxing agent.

The structure is fired in a gas-fired furnace to a maximum temperature of 1560 C. in a period of 108 hours. The furnace is cooled for 24 hours and the specimens removed. The fired product is white in color with thin translucent walls and weighs 785 grams. The compos1- tion of the fired product consists essentially of the aluminum silicate, mullite with minor amounts of corundum. and amorphous silica. A sheet-like void is present in the center of the walls approximately 0.003" wide. The separate walls surrounding this void are approximately 0.006" thick. In addition, the Walls contain a few isolated spherical voids ranging in diameter from about 10-40 microns.

Example 2 An aluminum honeycomb approximately 6" x 4" a: 1 /2" thick made of 0.003" AGC type aluminum foil. cell, obtained from Hexcel Corporation is etched in 1% caustic for several minutes. The final weight of the etched aluminum honeycomb template is 19.7 grams. The etched template is coated by immersing in a slurry consisting of 1 part by Weight of CMC solution of Example 1;

1 part by weight of sodium silicate solution of Example 1 part by Weight of aluminum powder of Example 1; and

1 part by weight of alumina of Example 1.

After coating, the structure is air dried and heat set as in Example 1. The dipping, drying, and heat setting cycles are repeated two times. The final dry weight of the structure is 131.2 grams. The coating, therefore. amounts to 111.5 grams or 5.6 parts by Weight per part of aluminum template, 0.9 part sodium silicate, 2.35 parts aluminum powder, 2.35 parts alumina. The unfired structure contains 1.23% Na O as fluxing agent.

The coated structure is fired as in Example 1 to a final weight of 180 grams. The product is all white in color and consists essentially of corundum. The walls are about 0.125" thick containing sheet-like voids approximately 0.003" wide. The separate Walls surrounding these voids are approximately 0.060" thick. The walls also contain a few isolated spherical voids from about 10-60 microns in diameter.

Example 3 Aluminum honeycomb templates approximately 2%" x 1 /2" x A" are' prepared from cell, 5052 aluminum alloy, 0.003" thick. The honeycomb cell axes are in the 1 /2 direction. The templates are etched for 45 minutes in a solution of aluminum etchant 33 obtained from AmChem Corporation, Ambler, Pennsylvania containing 10 grams of etchant in 700 m1. of water. The templates are coated in a slurry made up of:

1 part by weight of CMC solution of Example 1, and 1 part by weight of sodium silicate solution of Example 1.

9 Aluminum of Example 1 and alumina as shown in the table are added to the CMC and sodium silicate solutions. The alumina used in Items 1 to 4 is hydrated alumina C- 35 obtained from Alcoa, and the alumina used for Items the coating slurry used in Example 2. The coated tube is then air dried. The dipping and drying cycle is repeated 5 times. The dried unfired structure contains about 1.8 parts by weight of coating per part of aluminum in the tube.

TABLE FOR EXAMPLE 3 Components Added Ratio of Ingredients to CMC-Sodium Percent Flux in Unfired Structure Silicate Solution, Wt. of Coated Template, gr. of Total A1 pts. by wt. Aluminum A1 in coat. Alumina Template, First Second Third in Unfire gr. A1 dust A120 Coating Coating Coating Fired Structure Al in temp. Total A1 5 to 8 is the alumina of example 1. The aluminum honeycomb templates are coated by dipping into the slurry as in Example 1. Three coating treatments are applied. After each dipping the pieces are air dried and heat set as in Example 1. Firing is carried out as in Example 1. The fired pieces are white and consist essentially of corundum. The walls of all pieces contain sheet-like voids.

Example 4 A 6" x 6" x 1" section of aluminum honeycomb of 7 cell size and 0.001" aluminum foil of 5052 alloy is etched for one minute in 1% caustic solution. After etching, the dry weight of aluminum honeycomb is 22 grams. The etched aluminum template is coated by dipping into a slurry consisting of:

300 g. of CMC solution of Example 1;

1000 g. of sodium silicate solution of Example 1;

1200 g. of aluminum powder of Example 1;

2000 g. of fused enstatite (MgO-SiO -200 mesh obtained from Muscle Shoals Electrochemical Corporation, Tuscumbia, Ala.;

1816 g. of silica flour obtained from F oote Mineral Company, Paoli, Pennsylvania; and

1000 g. of water.

After dipping and drying, the coated template is dried and heat set as in example. The dipping, drying, and heat setting operations are repeated once. The final weight of the heat set, unfired structure is 276 g.

The coated structure is then placed in a gas-fired furnace in an oxygen containing atmosphere in accordance with the following schedule:

The final product is translucent white in color, strong, and hard; and when the sample is analyzed by powder X- ray technique, it is found to consist primarily of cordierite (ZMgO '2Al O -5SiO with a small quantity of amorphous silica.

Example 5 A cylindrical aluminum tube 12" in length and 3" in internal diameter and having walls 0.019" in thickness is etched in 1% caustic solution and coated by dipping in The coated tube is placed into a cold gas-fired furnace. The temperature of the furnace is raised at a constant rate to 1600 in a period of 60 hours and is held at this value for an additional 48 hours. The resulting tube has walls of approximately 4 in thickness. The walls have a dark grey appearance due to the presence of some unoxidized or partially oxidized aluminum.

The invention claimed is:

1. A process for making a thin-walled alumina-containing refractory structure which comprises the steps:

(1) providing an aluminum template structure having walls of about 1 to about 35 mils in thickness and being coated on all surfaces with:

(a) about A to about 12 parts by weight per part of aluminum in the template of a member of the group consisting of (1) aluminum powder and (2) mixtures of aluminum powder with finely divided filler refractory materials in which the weight of aluminum powder is at least 15% of the total weight of the mixture, and

(b) a fluxing agent in the amount of about 0.2 to 20% by weight based on the total weight of aluminum in the template structure and coating, said fluxing agent being selected from the group consisting of the oxides of the alkali metals, the alkaline earth metals, vanadium, chromium, molybdenum, tungsten, copper, silver, zinc, antimony and bismuth, precursors of these oxides and hydroxides of the alkali metals.

(ll) firing the structure in an oxygencontaining atmosphere at a temperature of at least about 660 C. until at least about 60 percent of the aluminum in the coated template is oxidized.

2. A process as defined in claim 1 wherein the walls of the aluminum template have a thickness of from 1 to about 10 mils.

3. A process as defined in claim 2 wherein the filler refractory is a member of the group consisting of magnesia, silica, and a mixture of magnesia and silica.

4. A process as defined in claim 2 wherein the filler refractory material is alumina.

5. A process as defined in claim 4 wherein the aluminum template structure is an aluminum honeycomb.

References Cited UNITED STATES PATENTS 2,977,265 3/1961 Forsberg et a1. 161-50 XR 3,112,184 11/1963 Hollenbach 156-89 XR 3,244,539 4/ 1966 Hare 106-62 XR 3,255,027 6/ 1966 Talsma 106-65 3,338,995 8/ 1967 Sowards 156-89 XR HAROLD ANSHER, Primary Examiner G. W. MOXON, Assistant Examiner US. Cl. X.'R. 

